The particle beam therapy is a method of treating a cancer by imparting to a tumor in the body of a patient an irradiation dose of charged particles such as protons or carbon ions accelerated to about several hundred MeV using an apparatus such as an accelerator. In the method, it is important to form a dose distribution that approximates as much as possible to a target dose distribution prescribed for the tumor by a doctor. The target distribution is, in many cases, uniform inside the tumor and as lower as possible outside the tumor than thereinside.
When an object (including a human body) is irradiated with a particle beam accelerated by an accelerator, a three-dimensional dose distribution is generally formed whose characteristic has a maximum dose peak at one point in the object. This maximum dose peak is referred to as “Bragg peak”. When a maximum dose peak exists at one point in a three-dimensional space, the peak point is defined as “irradiation position” of the particle beam. In order to form a three-dimensional target dose distribution using a particle beam having the above-described peak structure, some contrivance is needed.
One of methods of forming a target dose distribution is a scanning irradiation method. In order to employ the method, a mechanism such as electromagnets is basically used that arbitrarily deflects the particle beam in two directions, i.e., in the X- and Y-directions perpendicular to the Z-direction, the traveling direction of the particle beam. A function is further needed that arbitrarily varies in the Z-direction the position where the Bragg peak is formed, by adjusting the energy level of the particles. The accelerator, which is a particle beam generating apparatus, is generally provided with a mechanism of adjusting the energy level. A plurality of irradiation positions (also referred to as spots) are set in a tumor, and then each irradiation position is sequentially irradiated with the particle beam using the above two mechanisms. Dose balance to be imparted to each irradiation position is preliminarily adjusted so that summation of individual dose distributions imparted to each irradiation position resultantly forms a target dose distribution.
In general, it takes less than 1 msec to scan-shift the particle beam from an irradiation position to a next irradiation position by deflecting the beam in the X-Y direction, while it takes approximately 100 msec to shift the Bragg peak position in the Z-direction by varying the beam energy level. For that reason, the ordinary sequence of irradiating each irradiation position is such that all irradiation positions corresponding to an energy level of the beam are irradiated first with the beam of the energy level by scanning the particle beam in the X-Y direction, and then the energy level is changed to a next one.
When the Bragg peak position is shifted in the Z-direction by varying the energy level, irradiation with the particle beam must be always stopped, that is, the beam must be interrupted. The scanning irradiation method is classified into the following methods depending on the way of scanning in the X-Y direction.
A scanning irradiation method in which the particle beam is interrupted during scan-shifting from an irradiation position to a next irradiation position is called a spot scanning method or a discrete spot scanning method. For example, in the spot scanning method, a mechanism for measuring a dose imparted to each irradiation position is provided, and the method is implemented in such a way that the particle beam is once interrupted when a measured dose reaches a prescribed dose to be administered to an irradiation position, and then the particle beam is scan-shifted to a next irradiation position.
In a case of no interruption of the particle beam during scan-shifting from an irradiation position to a next irradiation position, a scanning irradiation method is classified into two methods. One is a method in which a mechanism is provided for measuring a dose imparted to each irradiation position, and the particle beam is scan-shifted to a next irradiation position without interrupting the beam at the time when the measured dose reaches to a certain value. This method is referred to as a raster scanning method (see, for example, Patent Document 1). Since the irradiation is continued during scan-shifting of the particle beam, the summation of a distribution of doses imparted during scan-shifting and that of doses imparted not during scan-shifting but during staying at irradiation positions is adjusted to a target dose distribution.
The other is a line scanning method in the case of no interruption of the particle beam during scan-shifting from an irradiation position to a next irradiation position. In this method, the irradiation target is irradiated with the particle beam by continuing scanning of the particle beam without staying at each irradiation position. Functions of keeping constant the beam intensity, which is a dose imparted per unit time, and of varying the scanning speed arbitrarily are provided for scanning the particle beam at a low speed near an irradiation position to which a high dose to be imparted and at a high speed near an irradiation position to which a low dose to be imparted. By controlling the scanning speed in this way to be inverse-proportional to a dose to be administered to each irradiation position, the resultant summation of a dose distribution is adjusted to be a target distribution.
In each scanning method above, although a target dose distribution should be obtained according to calculation, a dose distribution actually obtained may not be a target one since there are various uncertainties in practical irradiation. The uncertainties are caused such as by instability in the position and intensity of the particle beam, a positional error in fastening a patient, error in CT data of the patient, a signal delay in the control equipment, and noise, for example. Due to influence of these uncertainties, an actual dose distribution may differ from calculated one. Moreover, in a case of a tumor particularly in a liver or a respiratory organ such as a lung, it is difficult to impart an irradiation dose to the tumor in accordance with a treatment plan because the position of the tumor, conditions around the tumor and the like are changing temporally owing to respiration of the patient.
There has been a method referred to as “rescanning” or “repainting” for resolving the above problems (see, for example, Patent Document 2). In the method, each irradiation position is dividedly irradiated multiple times with the particle beam. The method is based on the concept that error in a dose distribution is canceled out and reduced by summing the multiple time irradiations. The number of divided irradiation is referred to as a rescan count. The sequence of irradiation is such that the particle beam of an energy level is scanned at first in the X-Y direction to irradiate once all irradiation positions corresponding to the energy level. After that, each irradiation position is irradiated again with the energy level remaining unchanged. The irradiation is repeated for a rescan-count number of times, and then the energy level is changed to a next one. The rescan count may be different for each energy level or may be the same for all energy levels. Generally, influence of the error is cancelled out and reduced more as the rescan count is increased.